The prodigal son was always the son, just as the elect have always been the elect children of God
Each one of the elect is like the prodigal son in this, that for a time he is deluded by the world and is led astray by his own carnal appetite. He tries to feed on the husks, but they do not satisfy. And sooner or later he is obliged to say, "I will arise and go to my father, and will say unto him, Father, I have sinned against heaven, and in thy sight."(repentance)
Notice that the text makes this comment
Luk 15:16 "And he would have gladly filled his stomach with the pods that the swine were eating, and no one was giving {anything} to him. Luk 15:17 "But when he came to his senses, he said, 'How many of my father's hired men have more than enough bread, but I am dying here with hunger!
He had a turning, a revelation and he "came to his senses".
And he meets with the same reception, tokens of unchanging love; and a father's welcome voice echoes through the soul, and melts the heart of the wayward sinful son ---"This my son was dead, and is alive again; and was lost, and is found."
Let it be noticed that this is a thoroughly Calvinistic parable in that the prodigal was a son, and could not lose that relationship.
This was a Parable, not a true event.. Jesus taught in parables not to make things clearer to everyone listening...but to those that have ears to hear...
This parable was pointed right at the heart of the self righteous law keeping Pharisees that thought their lives would commend them to God
He came to his senses, and returned to his father, who welcomed him, and said that the one who was dead is alive.
Can we repent? Can the lost repent? Can the dead repent?